1. Field of the Invention
The present invention relates generally to a diagnostic device that diagnoses a failure or malfunction of a differential pressure detecting unit which detects a differential pressure of an exhaust gas at upstream and downstream sides of an exhaust emission control device disposed in an exhaust system of an internal combustion engine.
2. Description of Related Art
As an exhaust emission control device of an internal combustion engine, a diesel particulate filter (called DPF) is, for example, disposed in an exhaust system of a diesel engine to reduce hazardous materials included in an exhaust gas of the engine. In this system, a differential pressure detecting sensor is disposed so as to detect a differential pressure of the exhaust gas between upstream and downstream sides of the DPF, and it is judged or diagnosed based on a detection result of the sensor whether or not a failure has occurred in the sensor. For example, Published Japanese Patent First Publication No. 2005-307880 has disclosed a diagnostic device. In this device, because particulate matters contained in the exhaust gas are deposited in a DPF, a quantity of the deposited particulate matters is estimated based on operation conditions of the engine, and a normal range of differential pressure detected by the sensor is determined based on the estimated quantity. Then, it is judged whether or not a detection result of the sensor is placed within the normal range. When the detection result is placed out of the normal range, the diagnostic device judges that a failure has occurred in the sensor.
However, there are many types of failure that can occur in the sensor. When a specific type of failure has occurred in the sensor, a detection result of the sensor having no relation to a differential pressure of the exhaust gas is sometimes placed or fixed within the normal range. In this case, even when the sensor is actually placed in a fault condition, the diagnostic device sometimes misjudges that the sensor is in a normal condition.